Ask them if they added a zone for 'forums.rapture-ot.com' with proper A and NS records.
But another thing you can do is to just make an A-Record for forums.rapture-ot.com to 220.127.116.11. (while hosting dns on theplanet still)
This way you won't delegate the domain to them, and seeing as they probably have a script to add a vhost in their webserver for this domain it should work.
But it should be said some features in the free hosting might not work.
Like you'll have to add MX records if oyu want [email protected] for instance.